Re: [PATCH] mm: Only force scan in reclaim when none of the LRUs are big enough.

[Rik van Riel]

On 03/15/2014 11:36 PM, Hugh Dickins wrote:
> From: Suleiman Souhlal <suleiman@xxxxxxxxxx>
>
> Prior to this change, we would decide whether to force scan a LRU
> during reclaim if that LRU itself was too small for the current
> priority. However, this can lead to the file LRU getting force
> scanned even if there are a lot of anonymous pages we can reclaim,
> leading to hot file pages getting needlessly reclaimed.
>
> To address this, we instead only force scan when none of the
> reclaimable LRUs are big enough.
>
> Gives huge improvements with zswap. For example, when doing -j20
> kernel build in a 500MB container with zswap enabled, runtime (in
> seconds) is greatly reduced:
>
> x without this change
> + with this change
>      N           Min           Max        Median           Avg        Stddev
> x   5       700.997       790.076       763.928        754.05      39.59493
> +   5       141.634       197.899       155.706         161.9     21.270224
> Difference at 95.0% confidence
>          -592.15 +/- 46.3521
>          -78.5293% +/- 6.14709%
>          (Student's t, pooled s = 31.7819)
>
> Should also give some improvements in regular (non-zswap) swap cases.
>
> Yes, hughd found significant speedup using regular swap, with several
> memcgs under pressure; and it should also be effective in the non-memcg
> case, whenever one or another zone LRU is forced too small.
>
> Signed-off-by: Suleiman Souhlal <suleiman@xxxxxxxxxx>
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>

Acked-by: Rik van Riel <riel@xxxxxxxxxx>

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